1. Field of the Invention
This application is a 371 of PCT/EP94/00155 filed Jan. 21, 1994.
This invention relates to a process for the production of .alpha.-branched aliphatic monocarboxylic acids containing 12 to 48 carbon atoms. In a first stage of the process (stage a), .alpha.-branched aliphatic monohydric alcohols (Guerbet alcohols) are converted into the alkali metal salts of the corresponding .alpha.-branched aliphatic monocarboxylic acids in the presence of alkali metal hydroxide and, in a second stage of the process (stage b), the .alpha.-branched aliphatic monocarboxylic acids are released from the alkali metal salts by soap decomposition.
2. Statement of Related Art
In the oleochemical industry, .alpha.-branched alcohols corresponding to the formula: ##STR1## in which R is a C.sub.4-20 hydrocarbon radical, are prepared from unbranched, saturated primary alcohols R--CH.sub.2 --CH.sub.2 OH by the Guerbet reaction.
These alcohols and their derivatives are used in a number of formulations in the lubricant industry, in cosmetics and in textile care.
In the case of the fatty acids, it has not hitherto been possible to produce similarly branched products on an industrial scale although there are corresponding potential applications for these fatty acids and their derivatives. The esters with polyhydric alcohols, such as pentaerythritol or trimethylol propane, are distinguished with particular advantage by low vapor pressures and low pour points.
Accordingly, there has been no shortage of attempts to find production processes for the branched fatty acids.
In a process of the type mentioned at the beginning which is described in DE-A-2 320 461, Guerbet alcohols are first reacted with alkali to form the carboxylic acid salts. The corresponding monocarboxylic acid is then prepared by soap decomposition with mineral acids. The first stage of this process is carried out in an oxidative alkali melt at correspondingly high temperatures. In view of the melting points of NaOH and KOH, which are used for economic reasons, operating temperatures of more than 370.degree. C. have to be applied.
The pour points of the soaps, which have to be present in liquid form according to this prior art so that thay may be separated from the reaction mixture, are also above 340.degree. C. The hydrogen formed during the reaction leads to safety problems in view of the elevated temperature. Besides blockages of pipelines, the highly corrosive effect of the alkali melt on virtually all known materials is a major obstacle to the operation of this process on an industrial scale.
In a process for the production of salts of carboxylic acids from the corresponding alcohols and a base, which is known from EP 31 694 B1, the formation of solid reaction products and the foaming of the reaction mixture during the alkaline oxidation of the alcohols are prevented by the use of inert viscosity reducers. A mixture of the base and a catalyst in an inert diluting liquid is introduced before the beginning of the reaction. The alcohol to be reacted is only added to this mixture thereafter under the reaction conditions. However, addition of the diluent before the reaction leads to a reduction in the volume/time yield to values of 20 to 80%.
The sole object of adding water, as mentioned in the Comparison Examples of EP 31 694 B1, is to decelerate the saponification reaction and hence the formation of hydrogen and thus to prevent foaming. The criterion mentioned in EP 31 694 B1 with regard to the choice of the diluent is the low volatility under the reaction conditions so that the diluent remains in the liquid phase.